The present invention relates generally to an automotive engine control system for controlling engine operation such as an ignition timing control, or fuel injection control. More particularly, the invention relates to a fail-safe system in the automotive engine control system for back-up of a crank angle sensor which serves as a control parameter signal generator for the engine control by producing a signal representative of an engine revolution parameter.
In an automotive engine control system, the engine revolution parameter is one of the most important parameters of the control process. For example, in spark ignition control, spark ignition timing and spark advance angle are determined mainly on the basis of the engine revolution parameter. Similarly, in fuel injection control, fuel injection timing is determined in synchronism with engine revolution. As is per se well known, engine revolution is detected by a crank angle sensor attached to an engine crank shaft for rotation therewith. Generally, the crank angle sensor produces a crank reference angle signal at predetermined angular positions of the crank shaft, for example every 120.degree. or 90.degree. and a crank position signal when the crank shaft rotates through a given angle, e.g., 1.degree. or 2.degree.. On the basis of the crank reference signal, and the crank shaft angular position, the times at which a reciprocating piston is at top dead center (TDC) and bottom dead center (BDC) are detected. On the other hand, based on the crank position signal, the engine revolution speed (hereafter referred as "engine speed") is detected.
Since the spark ignition timing and spark advance angle are determined on the basis of the crank reference signal as set forth previously, the spark ignition control may malfunction if the crank angle sensor fails to produce the crank reference signal for some reason. This may result in stalling of engine due to the absence of ignition in the combustion chambers. Similarly, the fuel injection control may malfunction when the crank reference signal is absent. In this case, since the fuel injection amount in the fuel injection control is generally determined on the basis of the engine speed and an engine load condition which is represented by an intake air flow rate, intake vacuum pressure, or the like, the absence of the crank position signal, i.e. the absence of the engine speed parameter, may cause the fuel injection control system to fail to determine the fuel injection amount.
If the crank reference signal and crank position signal are outputted from the crank angle sensor but contains some error, the ignition control and fuel injection control may operate erroneously.
In order to detect fault in the engine control system, there have been developed various monitor systems for monitoring the control operation and/or sensor signals providing the control parameters. For example, U.K. Prepublication No. 2,035,633, published on June 18, 1980 discloses a malfunction preventing system for a microcomputer system, in which abnormal conditions during program execution are detected to produce an alarm signal and/or reset the system to an initial position thereof. In addition, in the U.K. Prepublication No. 2,046,964, published on Nov. 19, 1980, which has been patented in the United States under U.S. Pat. No. 4,339,801, on July 13, 1982, discloses an electronic control system for an automotive vehicle provided with a plurality of check programs for checking the system operation. Furthermore, Japanese Patent Prepublication No. 56-141534 published on Nov. 5, 1981 discloses a monitor system for an engine control system in which sensor signals and actuator operations are checked during execution of the checking operation.
Although various check or monitor systems have been provided and/or developed conventionally, these may not satisfactorily and successfully prevent the engine control system from stalling if the crank angle sensor ceases to produce the sensor signals or produces erroneous signals.